When we first began RVing some 14 years ago friends of Ginny gave us a Jump Starter, something we would have never bought for ourselves. We felt it was valuable because we are often a long way from charging assistance.
It turns out that we have had occasion to use it a number of times over the years and more often to help others with it. (The same might be said for our compressor that we frequently use but has helped others a number of times.)
After 14 years our Jump Starter has worn out so Ginny said buy another. This time, with new battery
technology, we have a jump starter that will fit in a large pocket and best of all can be topped off while driving. (One of the problems with our rather large 1st Jump Starter was that you had to remember to occasionally top it off, not with this one that can be charged topped off from the 12 volt out let while driving.)
Amazon.com: PowerAll PBJS12000R Rosso Red/Black Portable Power Bank and Car Jump Starter: Automotive
Note this is not designed to start really large gas or diesel engines.